1. Field of the Invention
The present invention relates to an engine exhaust emission control device and exhaust emission control method, and in particular, to technology for purifying nitrogen oxides discharged from a vehicle engine, using ammonia as a reducing agent.
2. Description of the Related Art
As a device for purifying air pollutants discharged from an engine, in particular nitrogen oxides (referred to hereafter as “NOx”) in exhaust gas, using a secondary-treatment, the following SCR (Selective Catalytic Reduction) device is known. This SCR device is installed in an exhaust gas passage of an engine, and includes an injection nozzle that injects an aqueous solution of ammonia or a precursor thereof. The ammonia injected by this injection nozzle functions as a reducing agent, and reacts with NOx on a catalyst, to reduce and purify the NOx. Moreover, as an SCR device that addresses ease of storage of ammonia on a vehicle, the following device is known. This SCR device is provided with a tank that stores urea water as an ammonia precursor, and at the time of actual operation, injects the urea water supplied from this tank into the exhaust gas passage, so as to produce ammonia from hydrolysis of the urea using exhaust heat (Japanese Unexamined Patent Publication No. 2000-027627 (paragraph number 0013)). Generally, engine operating conditions such as engine speed and load are detected, and urea water in an amount corresponding to the detected operating conditions, is injected into the exhaust gas (Japanese Unexamined Patent Publication No. 2001-020724 paragraph number 0004)).
However, in the above SCR device there are the following problems. As a setting related to an engine operation, there is a case where operating characteristics of an engine part such as a fuel injection valve are set in order to particularly reduce a particulate emission amount. In such a setting, generally a NOx emission amount increases. If the SCR device is operating normally, the discharged NOx can be purified by a reduction reaction with ammonia. Based on such a setting that permits NOx emission to some degree, a case where an abnormality has occurred in an engine part and the composition of the exhaust gas has changed is assumed. In this case, if the urea water injection quantity is maintained at the normal level irrespective of an increase in the NOx emission amount, then the ammonia becomes insufficient with respect to the NOx, and unpurified NOx is discharged into the atmosphere. On the other hand, if the urea water injection quantity is maintained at the normal level irrespective of a decrease in the NOx emission amount, then not only will the urea water be consumed unnecessarily, but excessive ammonia will be generated and the surplus ammonia will be discharged into the atmosphere. Furthermore, it is assumed that an abnormality has occurred in the SCR device, and the urea water injection quantity has changed, or the ammonia content amount in the urea water (that is, the urea concentration) has changed. In this case, since the ammonia addition amount to the exhaust gas changes, the proportion of NOx and ammonia deviates from an optimal value, and the reduction reaction does not proceed well, so that the NOx removal rate will not meet requirements. When ammonia is excessively added due to an abnormality in the SCR device, the surplus ammonia is discharged into the atmosphere.